Automatic tire inflation systems (ATIS) have become a very popular tire maintenance tool in the heavy vehicle industry. For example, approximately one-third of new semi-trailers produced in the United States in 2012 were produced with an onboard ATIS. These systems have existed for over 20 years and are rapidly gaining acceptance and popularity.
However, a primary criticism of ATIS (as opposed to CTIS, which allow manipulation of tire pressures but are prohibitively costly for the commercial transport industry) is that they solve underinflation issues but do nothing to solve overinflation of tires. This failure to handle overinflation can negate some of the advantages that ATIS provides in the first place (reduced possibility of tire failure, reduced tire wear, even tire wear, increased fuel economy). A 2011 DOT study endorses ATIS as a tire maintenance solution but identifies overinflation as one of its two primary drawbacks.
Tires accumulate excess pressure during operation due to exposure to direct sunlight, increase in ambient temperature, friction from contact with the road, proximity to heat generated from brakes, etc. Commercially available ATIS technologies to date deliver a prescribed pressure to tires and prevent underinflation but they do not provide a means of pressure relief to solve this overinflation.
Commercial transport fleets have also identified overinflation as a problem that they would like solved. As a result of these fleet concerns and study criticisms ATIS providers have made significant efforts to develop pressure relieving mechanisms to add to their ATIS products.
It is commonly accepted in the heavy-duty vehicle industry that the ideal pressure for a semi-trailer tire varies by up to 20 psi between an empty state and a fully loaded state. As an example, a tire with prescribed cold pressure setting of 100 psi will perform optimally at that pressure with no payload, but at the same pressure and a payload of 40,000 lbs. it will have an increased footprint, thereby making more contact with the road, increasing tire fatigue and decreasing tire life. In order to maintain an optimal footprint, minimize tire fatigue and extend tire life, that tire under a full payload will require an additional +/−20 psi of pressure.
At the higher of these pressures however but in an empty state or without a payload, the tire serves as a more rigid conduit between the vehicle and the road and thereby creates significantly more stress on the vehicle's undercarriage—axles, suspension, etc. Simply put, for optimal vehicle and tire performance a typical semi-trailer desires approximately 20 psi greater tire pressure when loaded than when empty.
As another example, it is commonly accepted in the heavy-duty vehicle industry that when temporarily moving at low speeds and/or in off-road conditions it is desirable to have a lower tire pressure in order to create a greater footprint and improve traction, mobility and safety. Once again there can be a desirable pressure difference of approximately 20 psi between the cold highway pressure of a semi-trailer tire and the optimal soft terrain or off-road tire pressure.
Therefore, there is a need for a system that solves overinflation via some pressure relief mechanism. Moreover, there is a need for system that delivers a series of two or more prescribed pressures to tires based on certain pre-defined conditions, such as loaded or unloaded vehicle, highway or off-road terrain and the like. There is likewise a need for a system that delivers a pre-selected pressure to tires based on a certain condition, such as empty vehicle; another pre-selected pressure to tires based on a next condition such as loaded vehicle; yet another pre-selected pressure to tires based on a next condition such as soft or unstable terrain, and so on.